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Cell Phone Noise Filter
(Forensic Version Only)
Some cell phone systems can interfere with audio equipment despite their use of signals well outside of the audio spectrum. Their carriers operate in the Giga Hertz range (0.850 – 1.9 GHz). GSM (Global System for Mobile Communications) based cell phones send out their packets of data in short bursts. These RF (Radio Frequency) bursts (pulses) can be radiated into audio system front end amplification devices like bipolar junction transistors or field effect transistors found discretely or inside operational amplifiers. The non-linear nature of these devices coupled with the integrating effect of collector to base (in BJT based circuits) or drain to gate (in FET based circuits) Miller capacitance creates a parasitic AM (Amplitude Modulation) de-modulator circuit. Thus, the envelopes of these RF bursts of energy are de-modulated into the audio range of frequencies. The repetition rate of these RF bursts lie within the audio spectrum. Thus, they can be detected and heard in an audio signal chain that is not sufficiently shielded. Because these noise bursts become audible, they can render an audio signal pathway extremely noisy, contaminated with a “buzz” like sound and thus, often unintelligible. The Diamond Cut Cell Phone Noise Filter is designed to attenuate the noise found in this situation.
The Cell Phone Noise Filter
The filter has two modes of operation. The first and more basic mode is more automatic, but often less effective while the second mode (Matched Filter) is more discriminating. When the Matched Filter is not checked, the system automatically attempts to find the cell phone noise pulses and interpolate them out of the audio signal. When the “Matched Filter” box is checked, you will need to highlight a sample of one of the noise pulses in the time domain display of the software and then click on “Save Sample”. The system will then use a time domain technique of pattern matching to detect further pulses in your audio stream and then replace them with interpolated audio.
Automatic (Non-Matched Filter) Mode
Un-check the “Matched Filter” checkbox. Click on the Preview button and then adjust the ratio control until the pulses are detected and reduced in amplitude. Then, adjust the “Size” control for the best overall signal intelligibility. Size relates to the length of the noise burst and not its amplitude.
Matched Filter Mode
Check the checkbox labeled “Matched Filter”. Zoom-in on a chain of cell phone pulses in the time domain display. Then, highlight a single pulse event making sure to capture the entire event. Do not highlight a train of pulses, just a single impulsive event. Be sure not to highlight any of the voice signal surrounding the noise pulse or burst. Next, click on the “Save Sample” button of the Cell Phone Noise Filter dialog box. Zoom back out on the time domain display and then hit “Preview”. Adjust the Ratio for the best level of detection (noise reduction). Note that the “Avg Time” control is not active in Matched Filter Mode.
Note: This filter works best after your file has been up-sampled to 44.1 kHz, 16 bits.
(Forensic Version Only)
Some cell phone systems can interfere with audio equipment despite their use of signals well outside of the audio spectrum. Their carriers operate in the Giga Hertz range (0.850 – 1.9 GHz). GSM (Global System for Mobile Communications) based cell phones send out their packets of data in short bursts. These RF (Radio Frequency) bursts (pulses) can be radiated into audio system front end amplification devices like bipolar junction transistors or field effect transistors found discretely or inside operational amplifiers. The non-linear nature of these devices coupled with the integrating effect of collector to base (in BJT based circuits) or drain to gate (in FET based circuits) Miller capacitance creates a parasitic AM (Amplitude Modulation) de-modulator circuit. Thus, the envelopes of these RF bursts of energy are de-modulated into the audio range of frequencies. The repetition rate of these RF bursts lie within the audio spectrum. Thus, they can be detected and heard in an audio signal chain that is not sufficiently shielded. Because these noise bursts become audible, they can render an audio signal pathway extremely noisy, contaminated with a “buzz” like sound and thus, often unintelligible. The Diamond Cut Cell Phone Noise Filter is designed to attenuate the noise found in this situation.
The Cell Phone Noise Filter
The filter has two modes of operation. The first and more basic mode is more automatic, but often less effective while the second mode (Matched Filter) is more discriminating. When the Matched Filter is not checked, the system automatically attempts to find the cell phone noise pulses and interpolate them out of the audio signal. When the “Matched Filter” box is checked, you will need to highlight a sample of one of the noise pulses in the time domain display of the software and then click on “Save Sample”. The system will then use a time domain technique of pattern matching to detect further pulses in your audio stream and then replace them with interpolated audio.
Automatic (Non-Matched Filter) Mode
Un-check the “Matched Filter” checkbox. Click on the Preview button and then adjust the ratio control until the pulses are detected and reduced in amplitude. Then, adjust the “Size” control for the best overall signal intelligibility. Size relates to the length of the noise burst and not its amplitude.
Matched Filter Mode
Check the checkbox labeled “Matched Filter”. Zoom-in on a chain of cell phone pulses in the time domain display. Then, highlight a single pulse event making sure to capture the entire event. Do not highlight a train of pulses, just a single impulsive event. Be sure not to highlight any of the voice signal surrounding the noise pulse or burst. Next, click on the “Save Sample” button of the Cell Phone Noise Filter dialog box. Zoom back out on the time domain display and then hit “Preview”. Adjust the Ratio for the best level of detection (noise reduction). Note that the “Avg Time” control is not active in Matched Filter Mode.
Note: This filter works best after your file has been up-sampled to 44.1 kHz, 16 bits.